how could iron ions possibly get into river water?
Let me count the ways. Iron ore in runoff areas plus acid rain from auto exhaust producing nitrates of iron which are soluble Iron pipes in sewer systems plus drawdowns from sewer lagoons of treated sewage. Etching of circuit boards plus outfalls. Actual leaching of very slightly soluble iron salts or breakdown of those same salts in rocks by biological action (lichens) River water is usually heavy in dissolved minerals as opposed to lake water, which is usually higher in organics, so there are natural sources of occurrence as well as polluting sources of occurrence.
1 way is from igneous rocks(which contains iron) being dissolved by the river
